Electrophotographic imaging processes and techniques have been extensively described in patents and other literature (e.g., U.S. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 and many others). Generally, these processes employ a photoconductive insulating material which is prepared to respond to imagewise exposure with electromagnetic radiation to form a latent electrostatic charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a permanent record of this image.
Various types of photoconductive insulating materials are known for use in electrophotographic imaging processes. In many conventional electrophotographic elements, the photoconductive insulating material is in a single layer composition affixed to a conductive support.
In addition, various multi-active layer electrophotographic elements, i.e., those having more than one active layer, have been described in the art. One useful type of multi-active electrophotographic element is described in U.S. Pat. No. 3,165,405 to Hoesterey at column 2, lines 6-20 thereof.
As described in Hoesterey, photoconductivity is achieved by applying a uniform positive charge to the surface of an element containing two layers of zinc oxide (i.e. a sensitized zinc oxide bottom layer and an unsensitized zinc oxide upper layer) and then exposing the sensitized bottom layer to a pattern of activating radiation. Photoconductivity is produced in the element by the electrical interaction of the two zinc oxide layers. Specifically, the sensitized zinc oxide bottom layer generates photoelectrons, i.e. negative charge carriers, and injects these charge carriers into the unsensitized zinc oxide upper layer which accepts and transports these charge carriers to the positively charged surface of the photoconductive element. The former layer is typically referred to as a charge generation layer, while the latter is a charge transport layer.
The concept of using two or more active layers in an electrophotographic element, with at least one layer designed primarily for the photogeneration of charge carriers and at least one other layer transporting such generated charge carriers, has been discussed in the patent literature. In addition to the above-noted Hoesterey patent, these patents include: U.S. Pat. No. 3,041,166 to Bardeen; U.S. Pat. No. 3,394,001 to Makino; U.S. Pat. No. 3,679,405 to Makino et al.; U.S. Pat. No. 3,725,058 to Hayaski et. al.; U.S. Pat. No. 4,175,960 to Berwick et al.; U.S. Pat. No. 4,618,560 to Borsenberger et al.; U.S. Pat. No. 4,719,163 to Staudenmayer et al.; Canadian Patent No. 930,591 issued Jul. 24, 1973; Canadian Patent Nos. 932,197-199 issued Aug. 21, 1973; and British Patent Nos. 1,337,228 and 1,343,671.
Various shortcomings still exist in multi-active electrophotographic elements. For example, the multi-active elements of the Hoesterey patent suffer from the disadvantages of low speed and difficulty in cleaning. Other multi-active elements, such as those described in Canadian Patent Nos. 930,591 and 932,199, are primarily designed for positive charging and, therefore, may not be suitable for electrophotographic processes where a negative charging mode is employed.
The charge-transport layer utilized in multi-active electrophotographic elements has employed a wide variety of charge-transport materials. Most charge-transport materials preferentially accept and transport either positive charges (i.e. holes) or negative charges (i.e. electrons). Transport materials which exhibit a preference for conduction of positive charge carriers are referred to as "p-type" transport materials, while those which preferentially conduct negative charges are referred to as "n-type". In a few cases, transport materials are capable of transporting either electrons or holes. These are referred to as "bipolar" transport materials.
N-type transport materials, used in conjunction with positively charged toners, are particularly suited to "neg-pos" imaging. Neg-pos imaging is also known as discharged area development ("DAD"), wherein the light-struck (discharged) regions of the element are developed with the positive toner. Neg-pos imaging is commonly performed by laser or light-emitting diode (LED) printers. Examples of n-type transport materials are disclosed in U.S. Pat. Nos. 4,277,551, 4,609,602, 4,719,163, 4,948,911, 4,175,960, 4,514,481, 4,474,865, 4,546,059, 4,869,984, 4,869,985, 4,909,966, 4,913,996, 4,514,481 and 4,921,637.
N-type transport materials generally suffer a number of drawbacks, including low mobility of the photoelectrons, low electrophotographic speeds, high dark conductivity, poor solubility in or compatibility with binder materials, and Poor mechanical properties such as brittleness of the coating.
P-type charge-transport materials, used in conjunction with positively charged toners, are commonly used for "pos-pos" imaging. Pos-Pos imaging is also known as charged area development ("CAD"), wherein the unexposed and charged areas of the element are developed with positive toner Pos-Pos imaging is commonly seen in conventional optical copiers. Representative p-type charge-transport materials include carbazole materials, arylamine-containing materials and polyarylalkane materials. These and other illustrative p-type charge-transport materials are disclosed in Staudenmayer et al., cited above. Although, p-type charge-transport materials are superior in many respects to n-type materials, a continued need for improvement remains.
The multi-active elements described, for example, in U.S. Pat. Nos. 4,719,163, 4,618,560, and 4,175,960 disclose embodiments using only a p-type or an n-type charge transport material. Therefore, electrophotographic apparatus employing such elements are restricted to either neg-pos imaging or pos-pos imaging and cannot perform both.
Bipolar charge-transport material offers the advantage of an electrophotographic element capable of performing either neg-pos or pos-pos imaging by simply changing the polarity of the applied charge. This allows one machine to act as both an optical copier and an electronic printer. However, bipolar charge-transport materials currently available suffer many of the same disadvantages as n-type charge-transport materials in that they typically comprise a mixture of n-type and p-type charge-transport material.
Free radicals (i.e. an atom or group of atoms possessing an unpaired electron) have been known to have photochemical and photoconductive properties as disclosed in Eley et al., Semiconductivitv of Organic Substances, 63 Trans. Faraday Society 902-910 (1967) and Bogatyreva and Buchachenko, Photochemical Investigation of Perchlorotriphenylmethyl Radicals, 15 Kinetika i Kataliz 1152-1157 (Sep.-Oct. 1974). In electrophotographic systems, such compounds have been used to form electrostatic images by changing the conductance of light-sensitive reproduction sheets. This is disclosed in U.S. Pat. No. 3,600,169 to Lawton, Japanese Patent No. 114015 issued Jul. 15, 1977, and Japanese Patent No. 072387 issued Jan. 14, 1974. Stable free radicals have been used as sensitizers to increase the photosensitivity of photoresponsive compositions, as disclosed in U.S. Pat. No. 3,434,833 to Fox. However, free radical compounds have not been used as a charge-transport material in the charge transport layer of a multi-active electrophotographic element.